The present invention relates to an eye examination apparatus.
The apparatus according to the invention is preferably used in ophthalmology to acquire images of the retina. In general, however, it can be used to acquire images of any object.
The use of apparatus for eye examination, commonly defined with the term “fundus cameras”, is widely known. In general, these apparatus illuminate the whole of the retina with a flash of white light and acquire the light reflected by the retina to produce color photographs of this latter.
Usually, apparatus of this type, operating with the pupil not ciliated (non-mydriatic), are capable of photographing a relatively limited portion of the retina, corresponding to a maximum field of view of 45°, in conditions with pupil diameters of at least 4 mm. Some apparatus are also able to produce infrared videos and photographs.
The images (color or infrared) that can be obtained by illuminating the whole retina simultaneously are typically characterized by low contrast, due to the partial transparency of the retina. This drawback is more pronounced in the case of infrared videos or photographs.
Eye examination confocal apparatus are known. These apparatus can produce photographs and videos of the retina with good contrast and field of view greater than 45° (also in conditions with pupil diameters of less than 4 mm). There are two types of known confocal apparatus, i.e. the point scanning type and the line scanning type. For reasons of simplicity of construction, confocal line scanning apparatus are of particular industrial and commercial interest.
Examples of confocal line scanning apparatus are described in the patent documents U.S. Pat. No. 4,241,257, U.S. Pat. No. 7,331,669 and EP2392915A1.
These apparatus scan the retina with a light beam that illuminates a very narrow region of the retina, typically in the form of a line. They collect and de-scan the light reflected by the retina, passing it through a fixed diaphragm in slit form that eliminates most of the spurious light coming from undesired reflections. The light passing through the diaphragm is then projected on a sensor, by means of which one or more images of the retina are acquired.
An important drawback of confocal line scanning apparatus consists in that they require illumination sources with high radiance to obtain satisfactory illumination power to acquire the photos. Consequently, the light sources typically used consist of lasers, superluminescent diodes (SLD) or the like. These light sources are generally monochromatic or have a limited light emission band. For this reason, these apparatus are capable of supplying high contrast retinal images only with infrared or monochromatic visible light.
Some apparatus are also able to supply color images of the retina by combining monochromatic images obtained used red, green and blue lasers as light sources. However, the color images thus obtained do not offer the same degree of detail and natural color tones as images obtainable with an examination apparatus that uses a light source capable of emitting white light.
Examples of confocal line scanning apparatus that use white light sources have been proposed. However, due to insufficient radiance of the available white light sources, the solutions proposed to date have not led to mass-produced commercial products.
The apparatus in the U.S. Pat. No. 7,331,669 describes a digital photographic system that scans the retina with the light generated by an illuminator containing a diaphragm in the shape of a slit to reduce undesired reflections. In one of the construction variants proposed, the light reflected by the retina is de-scanned, passed through a confocal diaphragm that eliminates reflections and then rescanned to be projected on a two-dimensional sensor.
The photographic system described in this patent document does not provide practical solutions to improve, in a satisfactory manner, the transfer of power from the light source to the retina, so as to enable the use of low radiance white light sources, such as LED devices or the like. Consequently, the two-dimensional sensor must have a high gain during acquisition. This often causes the images acquired to be noisy, above all at the edges.
The increased exposure time using a slow scan, as proposed in the U.S. Pat. No. 7,331,669, improves the signal/noise ratio of the images. However, this solution increases the risk of movement of the eye during acquisition of the images, with the possible consequent presence of movement artefacts and blurring.
The patent application WO2009004497A2 proposes a white light apparatus provided with a linear source produced as an incandescent wire or a strip of LEDs, suitably arranged. The solution proposed for the shape of the light source does not solve the problem of insufficient radiance of this latter. Therefore, it does not cause an increase in the light power transmitted in the region of the retina (in the form of a line) that is illuminated.
A further drawback of prior art confocal line scanning apparatus consists in the fact that they do not typically allow photographs with sufficiently uniform distribution of brightness to be obtained, above all in the case of images corresponding to fields of view greater than around 30°.
It is known how common LED or laser diode devices emit light with generally non-uniform power distribution, variable as a function of the light emission angle. The power density of these light sources is usually greater along the optical axis and progressively decreases toward the periphery of the light emission cone. This behavior makes it difficult to construct a collimating optics of the light source that supplies an output light beam having uniform power density.
It must also be considered that the retina of the eye typically has reflectivity variable as a function of the illumination angle and type of light used for illumination (e.g. white or infrared). This makes the distribution of brightness in the images of the retina even more non-uniform.
Any correction of an image acquired with non-uniform light distribution via software worsens the signal/noise ratio in the regions characterized by low brightness. This reduces the quality of the images due to the increase in noise, making the images of little use for medical diagnosis.
In prior art examination apparatus of the line scanning type, one of the most widely used solutions to reduce reflections caused by the cornea and lens of the eye is to create a separation of the “side-by-side” type of the illumination light beam entering the eye with respect to the beam of light reflected by the retina and used by the apparatus to create the image of the retina. This solution has considerable advantages in terms of simplicity of construction and allows good quality images to be obtained for fields of view up to around 30°.
If images with greater fields of view, for example of around 60°, are required, the fact of having light beams passing through different regions of some lenses of the apparatus causes different distortions in the aforesaid light beams, preventing the line of light projected on the retina to be optically conjugated, for the whole of its length, with the confocal diaphragm. This causes a loss of brightness or the appearance of dark regions toward the outer edges of the images.
Further examples of eye examination apparatus are disclosed in patent documents U.S. Pat. No. 5,847,805 and US2005/068497.